Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for Manufacturing a Strip of Aluminium for Lithographic Printing
Plates
The invention relates to a process for manufacturing a strip of aluminium or
an
aluminium alloy for electrolytically roughened lithographic printing plates,
where-
by the metal is continuously cast to strip form and the cast strip is
subsequently
cold rolled to final thickness (de). Also within the scope of the invention
are litho-
graphic printing plates with electrolytically roughened surface.
Lithographic plates of aluminium, which are typically about 0.3 mm thick,
exhibit
advantages over plates made of other materials - of which only some are
mentioned here viz.:
- A uniform surface which is well suited for mechanical, chemical and electro-
chemical roughening.
A hard surface after anodising - which enables a large number of copies to
be printed.
- Light weight
- Low manufacturing costs
The publication ALUMINIUM ALLOYS AS SUBSTRATES FOR LITHOGRAPHIC
PLATES by F. Wener and R.J.Dean, 8t" International Light Metal Conference,
Leoben-Vienna 1987, provides an overview of the manufacture of strips for
litho-
graphic printing plates.
Today lithographic plates are mainly manufactured using aluminium strips which
are produced from continuously cast ingots by hot and cold rolling and inter-
mediate annealing (heat-treatment). In recent years various attempts have been
made to process strip cast aluminium materials to lithographic plates.
In EP-A-0 821 074 a process for manufacturing an aluminium strip for
lithographic
plates is described whereby the metal is continuously cast in the roll gap
between
cooled rolls of a strip casting machine and then rolled to final thickness
without
intermediate annealing. Often, the specifications for lithographic sheets
state
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maximum values for strength which can be achieved without inter-mediate
annealing only when the thickness of the cast strip is much less than 3 mm. In
practice, however, with conventional strip casting machines such small
thicknesses of cast strip are not easy to produce with good quality.
Known from EP-A-0 653 497 is a process for manufacturing an aluminium strip
for lithographic printing plates in which first a cast strip of max-imum
thickness
3mm is likewise produced using a roll caster. The cast strip is subjected to
an
intermediate, recrystallising anneal during cold rolling. This anneal is
carried out -
at a temperature of at least 300°C and a heating-up rate of at least 1
°C /sec -
stationary in coil form in a furnace, preferably at temperatures of 400 to
550°C in
a continuous anneal furnace. After this anneal the cast strip is cold rolled
directly
to a final thickness of 0.5 mm.
On cold rolling aluminium strips it is also known, in particular with
reductions
exceeding 90 %, to perform a recrystallisation intermediate anneal in a temper-
ature range of normally 300 - 400°C.
The object of the present invention is to provide a process of the kind
described at
the start which, without costly equipment, results in a lithographic strip
with good
strength also after a stove-lacquering cycle on lithographic printing plate
manufactured from the strip.
That objective is achieved by way of the invention in that:
a) the metal is cast to a maximum thickness of 4.5 mm,
b) without further heating, the cast strip is rolled to an intermediate
thickness
which corresponds to 30 to 80 % of the total reduction in thickness,
c) the strip rolled to intermediate thickness is annealed in a temperature
range
of 250 to 320°C in such a manner that at low strength recovery takes
place
without recrystallisation occurring, and
d) after the intermediate anneal the strip is rolled to final thickness
without
further heating.
Here the expression "Without further heating" means that, between the point of
leaving the roll gap of the casting machine and rolling to intermediate
thickness,
the cast strip receives no further external heating. If the cast strip, which
after
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leaving the roll gap of the casting machine still exhibits a relatively high
temp
erature for a certain length of time, is rolled to intermediate thickness
shortly after
casting, then the starting temperature for rolling, especially for large strip
thick
nesses, can be increased. For low strip thicknesses the processing to
intermediate thickness corresponds to cold rolling.
The essential aspect of the process according to the invention lies in the
intermediate annealing which serves the purpose of achieving recovery in the
structure and not the creation of new grains as is the case with the normal
recrystallisation intermediate annealing according to the state-of-the-art
procedures.
Aluminium strips subjected to the annealing treatment according to the
invention
undergo a smaller loss in strength after a stove lacquer cycle than strips
that have
been subjected to a recrystallising anneal.
The process according to the invention results, therefore, in lithographic
printing
plates which, also in connection with high stove lacquering temperatures of up
to
300°C, offer advantages with respect to final strength over
conventionally manu
factured lithographic sheets.
The metal is preferably cast with a maximum thickness of 3.5 mm, in particular
2.0 to 3.0 mm, advantageously 2.4 to 2.8 mm. The cast strip obtains thus an
ideal
microstructure in a region close to the surface which, in combination with the
recovery anneal according to the invention, results in strip rolled to final
thickness
with a surface structure exhibiting excellent etching behaviour.
Basically any strip casting method may be employed to produce the cast strip,
whereby, ideally rapid solidification accompanied by warm forming in the roll
gap
is desired. Both of the properties just mentioned are achieved e.g. by the
roll-
casting process in which the metal is cast into strip form between cooled
rolls. As
a result of the further processing of the cast strip by cold rolling and non-
recrystallising intermediate annealing the advantageous structure of the strip
near
the surface is retained due to rapid solidification.
The continuous casting method enables, at the same time, high solidification
rates
and very fine grain sizes in the regions close to the surface as a result of
dynamic
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recovery immediately after the cast strip emerges from the roll gap of the
casting
machine.
The further processing of the cast strip takes place by winding the cast strip
into a
coil of the desired size. In the next process step the strip is rolled to the
desired
intermediate thickness in a cold rolling mill suitable for lithographic sheet,
and
after the recovery anneal rolled to final thickness in the normal range of
about 150
to 300 wm.
The intermediate thickness at which the recovery anneal is carried out, the
temp
erature and the duration of annealing is selected on the one hand with respect
to
the initial thickness of the cast strip and on the other hand with respect to
the
composition of the material being processed. By means of a simple series of
trials, however, the expert in the field is able to determine without
difficulty the
parameters necessary to achieve the desired recovered condition.
The cast strip is preferably rolled to an intermediate thickness corresponding
to at
least 50% of the total reduction in thickness, whereby the suitable
intermediate
thickness is approximately 1.0 to 1.6 mm.
The recovery anneal of the material rolled to intermediate thickness
preferably lies
in a temperature range of 260°C to 300°C, usefully in a
temperature range of
about 270 to 290°C, whereby the strip rolled to intermediate thickness
is annealed
for a duration of about 2 to 5 h.
Apart from the advantage of uniform etching behaviour, a strip processed
accord-
ing to the invention exhibits excellent mechanical properties e.g. a high
strength
which falls only slightly during the stoving of a photosensitive lacquer
during the
production of lithographic plates.
The strip manufactured according to the invention is equally suitable for
etching in
HCI and HN03 electrolytes, whereby the advantages of the microstructure
achieved stand out especially when etching in an HN03 electrolyte.
All aluminium alloys used for the production of lithographic printing plates
may be
employed for manufacturing the lithographic strip according to the invention.
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Particularly preferred are alloys from the following series viz., AA lxxx, AA
3xxx or
AA 8xxx for example the alloys AA 1050, AA 1200 or AA 3103.
The above mentioned advantageous microstructure in the region close to the
surface of the strip is essentially the result of rapid solidification at the
surface. As
a result of the high solidification rates, the secondary phase particles
precipitate
out in the microstructure in a very fine form and high density . These
particles act
as first sites to be attacked on etching, in particular if the electrolytic
roughening is
carried out in an HN03 electrolyte. When the solidification rate at the
surface is
fast, the above mentioned particles exhibit an average spacing of less than
5pm
and form an interconnected network of uniform points that are attacked on the
surface. The actual three-dimensional roughness pattern begins to grow,
starting
from this first, very dense points of attack that are distributed uniformly
over the
whole surface of the strip. The small size of the above mentioned
intermetallic
phases has the further advantage that the time require for the electrolytic
dissolution at the start of etching is considerably reduced, with the result
that
electrical energy can be saved. As a result of the rapid solidification in the
surface
region non-equilibrium phases are formed by preference; the dissolution rate
of
these fine particles is high.
A further important microstructural feature of the strip manufactured
according to
the invention is the small grain size which is produced in the surface region
during
strip casting. The high density of points at which the grain boundaries
intersect the
surface, together with a high density of defects in the grains them-selves,
leads to
chemically active points of attack for continuous formation of etching pits.
During electrochemical etching, the microstructure in the strip surface
described
above results in the uniform roughness pattern required of lithographic
printing
plates. The advantages resulting from the use of the strip manufactured
according
to the invention are as follows:
- Uniform etching pattern as a result of a high density of potential points of
attack at the surface
- Etching in an HN03 electrolyte under critical electrochemical process
conditions
- Extension of the etching parameters into the range of low density of charge
and thus savings in energy
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- Prevention of etch defects in HN03 electrolytes as a result of undesirable
passivation reactions
- Formation of a dense network of cracks in the oxide layer in the passivation
range of the anodic potential as a result of a high density of intermetallic
particles of non-equilibrium structures
- Formation of a dense network of defects in the natural oxide layer in the
passivation range of the anodic potential as a result of a small grain size
with
many points of intersection with the oxide layer.
An important property of the lithographic sheet rolled to the desired final
thickness
of 0.2 to 0.3 mm is obtained from the subsequent process step viz., the
electro-
chemical roughening, which should achieve an etch structure on the surface
that
is as uniform as possible. To that end electrolytes of dilute hydrochloric
acid (HCI)
and electrolytes of dilute nitric acid (HN03) are employed which, depending on
the
type of plate required, produce a characteristic etch pattern under the
influence of
an alternating current.
If the etching is performed in a nitric acid electrolyte, it is found in
practice that a
uniform etch pattern is obtained only if certain etching parameters are
observed. If
e.g. for reasons of costs too low a charge (Coulomb) is employed, the etch
pattern
is usually irregular - in the form of streaks where no etching has taken
place. If
etching is carried out under these critical conditions, all the fine
differences in the
grain structure of the substrate (lithographic strip) become evident and it is
possible to classify the lithographic material used.
The reason for the sensitivity of the HN03 electrolyte to the electrolytic
etching
behaviour of aluminium lies in its passive range (passive oxide) and the
related
difficulty in initiating etching pits. Only at an anodic potential of +1.65V
(SCE) is
this passive range overcome by the formation of etching pits, whereas the form-
ation of pits in HCI begins already at a corrosion potential of -0.65V (SCE).
For
anodic etching in HN03 electrolytes this has the result that the intermetallic
phases are dissolved first in the potential range of -0.5 to -0.3 V (SCE),
before
the aluminium matrix is attacked and pitting occurs. The distribution of these
intermetallic phases forms a first network of pits on the etched surface; for
that
reason the areal density of these phases on the surface is important.
As explained above, the strip manufactured according to the invention, with
inclusion of a recovery anneal, exhibits the advantage over strip material
which
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has undergone a recrystallisation anneal, in particular in the case of
lithographic
printing plates after a stove-lacquer cycle at temperatures in the range of
approx.
270 to 300°C.
The advantage of the strip material manufactured according to the invention
over
a strip material with conventional recrystallising intermediate anneal is
revealed in
the following table which shows strength values of the alloys AA 1050 and AA
1200 at final thickness in the cold rolled condition and after various
simulated
stove-lacquer treatments.
The starting material for the investigations was a 4.5 mm thick strip produced
on a
roll caster machine. This strip was cold rolled to an intermediate thickness
of 1.5
mm and after intermediate annealing, cold rolled further to a final thickness
of
0.28 mm
The following intermediate anneal conditions were employed:
R (Recrystallising anneal) 380°C x 2h
E (Recovery anneal ) 300°C x 2h
The details of temperature and duration refer to the metal temperature and
duration of annealing after the strip has been heated with a heating rate of
100°C/h to the annealing temperature. The strength at failure (Rm) was
taken as
the strength characteristic.
The stoving of a photosensitive lacquer was simulated by immersing in a salt
bath
for the duration of 10 min.
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Table 1
Alloy Intermediate Stove-lacquer cycleStrength at failure
anneal
Rm MPa
R 157
E 177
AA 1050 R 132.8
240C x 10 min
E 170
R 129.0
260C x 10 min
E 158.2
R 115.4
280C x 10 min
E 140.0
R 91.3
300C x 10 min
130.7
R 179
E 181
AA 1200 R 136.2
240C x 10 min
E 155.1
280C x 10 min 9
25 3
R 93.6
300C x 10 min
E 103.4
15
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